Said water and/or oil-impermeable sealing mat consists of a flexible substrate layer, a layer of swellable clay, preferably bentonite, and a flexible cover layer.
In the past, bentonite was widely used in various forms to act as a water barrier. So it is already known to provide seepage resistant structures by employing a mass of swellable bentonite across the path of possible seepage or flow. One such method and composition for impeding the seepage or flow of water is disclosed in U.S. Pat. No. 2,277,286 (Bechtner). As therein more fully described, commercial bentonite is used to block leakage or flow of water seepage, and structures of various types are safeguarded against leakage by blocking the path of flow of the water with bentonitic or highy colloidal clay which possess the capacity to swell and gelatinize upon contact with water. One of the clays found best suited for this purpose was the true bentonite obtained in regions of Wyoming and South Dakota, although colloidal, or bentonitic clays which possess the property of swelling and gelatinizing in water to a substantial degree are also useful.
Also in the past, finally divided bentonite was fabricated into a laminate with water absorbent paper sheeting as described for example in the U.S. Pat. Nos. 4,048,373, 4,070,839 and 4,139,588. Such laminates are especially developed for use as a water barrier and are made from corrugated paperboard sheets.
The channels in said corrugated paperboard sheets are filled with finely divided bentonite. When such pannel is placed at the bottom of the pond and is exposed to water, the water passes through the top layer of kraft paper and is adsorbed into the bentonite material. At the same time the paper loses its tensile strength due to the wetting process. The bentonite has the capacity to expand and swell in response to absorbing the water. This expansion of bentonite and the loss of tensile strength of the kraft paper cause the bottom sheet member and the cover sheet member to no longer hold together.
Another way to package bentonite material in sheets or rolls, which can be placed on the bottom of the pond or lagoon so as to form a waterproof barrier thereon, is disclosed in U.S, Pat. No. 4,501,788 (corresponding to European Patent 0,159,625). In said Patent is described a method, for providing such a packaged bentonite sheet material utilizing the following process steps:
(a) Using a support polyester sheet material (for example a porous non-woven fabric) having the ability to permit gases to escape therethrough in a lateral direction. PA1 (b) Applying an adhesive to the upper surface of this sheet material, the adhesive being formed from a starch-like glue. PA1 (c) Applying approximately one-forth inch of bentonite on top of the adhesive. PA1 (d) Spraying a second coat of adhesive over the top of the bentonite. PA1 (e) Placing a scrim or fine mesh material on top of the adhesive. PA1 (f) Press rolling the above combination into a elongated flat sheet material. PA1 (g) Baking the sheet material in a long oven at approximately 300.degree. F. so as to bake all the moisture out of the sheet material and the bentonite. PA1 (a) Using a flat polyester sheet material, preferably a synthetic non-woven fabric which is a porous, flexible polypropylene material. The sheet material is capable of dissipating gas in a lateral direction so as to permit gas which gathers adjacent the sheet material to pass laterally outwardly through the sheet material. PA1 (b) Applying approximately one-fourth inch of bentonite over the top of the base material. PA1 (c) Applying plain kraft paper or other biodegradable material over the top of the bentonite. This material must be capable of degrading after hydration. PA1 (d) Stitching the sheet material to the base material with the bentonite being positioned between the two sheets of material. In the preferred form the stitches extend in crossing diagonal lines with respect to the longitudinal axis of the sheet material so as to form diamond shaped quilted compartments between the upper sheet material and the base sheet material. The quilted compartments contain bentonite therein. The quilted arrangement prevents the bentonite from shifting during the rolling of the quilted material and during transportation. In another form the kraft paper is corrugated so as to form elongated corrugated compartments for containing the bentonite material.
Not only is the above process cumbersome, expensive and time consuming, but also the support sheet and the cover sheet lose their ability to stick firmly to each other when moistened. This is a very important disadvantage, because the bentonite layer acts in a wet condition like a sliding path on the sides of the pond or other places. This sliding effect is further enhanced by the dissolved adhesive in the wet condition.
Therefore others have tried by a further development to avoid at least one of the before mentioned shortcomings, namely avoiding the use of an adhesive and the necessary baking process which uses such an adhesive. Such a new process which does not require baking or adhesive as above mentioned is disclosed in the U.S. Pat. No. 4,565,468. The process of said patent involves the use of the following steps:
When the above material is placed within a water environment, such as at the bottom of a pond or lagoon, the bentonite expands and breaks the kraft paper layer at the top of the barrier. The bentonite continues expanding so as to cover the stitch holes formed by the stitching, and thereby forms a water impervient layer.
As seen from the above description it may be that the process for the production of the sheet material according to the U.S. Pat. No. 4,565,468 is better than according to the process of the U.S. Pat. No. 4,501,788 (corresponding to European Patent 059 625), but there is still the large disadvantage that the bentonite layer during the use as a water barrier in a wet condition acts like a sliding path on slopes.
All of the sealing mats described in the obove U.S. patents serve merely to "package" bentonite and always consist in principle of a substrate layer, a bentonite layer and a cover layer. After these sealing mats have been laid out and subsequently moistened, the substrate and cover layers are connected only via the swollen bentonite layer therebetween, which has the consistency of grease. Now if it is considered that the sealing mats must further be weighted down with a sand or soil filling and then with gravel or rocks not only on flat surfaces, but also on slopes, it is easily conceivable that such a filling on the swollen intermediate bentonite layer, which acts like a slide, slips off, which is often observed in practice.
Thus, in principle, the sealing mats described in the abovegiven U.S. patents--as already indicated--serve only to pack the bentonite in flat form, such that the cover layer disconnects from the substrate layer upon the swelling of the bentonite and a continuous bentonite layer takes shape.
However, in reality such a bentonite layer can be produced more simply and inexpensively in situ in the manner described in the U.S. Pat. No. 4,344,722. Said patent provides a method and a system for waterproofing a desired substrate and further contemplates a waterproof and chemical-resistant product. The method comprises providing a length of flexible moisture-permeable thin, synthetic sheet material having desired characteristics, placing in contact with the substrate to be waterproofed a layer of the material, covering the layer of material with a central layer of bentonite (Montmorillonite clay) and placing on top of the bentonite a third layer of the fabric. The flexible moisture-permeable thin, synthetic sheet material is typically a non-woven fabric.
All of said prior sealing mats described above have the great disadvantage that the bentonite interlayer sandwiched therebetween can freely expand in all three dimensions when moistened, which results in a bentonite sliding plane.
Therefore applicant already tried to avoid the above mentioned disadvantages of the prior art with a sealing mat described in U.S. Pat. No. 5,041,330 which comprises a substrate layer, an interlayer of swellable clay, and a cover layer, in which at least one of the substrate layer and the cover layer consists of non-woven textile material; the other one, if any, of the substrate layer and the cover layer consisting of woven or knitted fabric and in which all three layers are bonded together by needle punching.
In the manufacture of such sealing mats, first of all, the granulated or powdered swellable clay is applied to the substrate layer and then the cover layer of non-woven sheets, i.e. a layer of needle-punchable fibers, is applied to the granulate or powder layer. These needle-punchable fibers are engaged by downwardly directed barbs arranged laterally of the shaft of the needle which are used for instance during the fabrication of non-woven sheets. The fibers which are thus engaged by the needles during needle punching are deflected in a direction perpendicular to the plane of the surface of the cover layer. This requires a certain strength and pliability of the fibers, and in the case of fibers an adequate staple length, in the case of endless filaments, for instance a deposition in the form of loose loops or coils, so that such loops can be downwardly drawn by the barbs without tearing the filaments.
As is known from the needle punching art a multiplicity of needles simultaneously carry out a needle puncturing operation and throughout a number of successive strokes, so that there results for instance a stitch density of 60 stitches per cm.sup.2 from the side of the cover or top layer into the support layer. The barbs of the needles, upon passage through the cover layer, tear along individual fibers or entire clusters of holding fibers and partially orient them to a point past the support layer. Upon retraction of the needles the holding fibers are stripped off of the barbs. The holding fibers then for the most part remain anchored over part of their length in the cover layer, whereas another part of the length of the same fibers is fixedly retained by the support layer. Owing to the multiplicity of needle stitches there are drawn-in such holding fibers at numerous locations distributed over the entire surface. Consequently, there are thus held together the support layer and the cover layer. On the other hand, the layer of particles has drawn therethrough numerous fibers, so that the particles are prevented from any lateral shifting in the plane of the surface of the article. The particles are embedded in a loosened form between the holding fibers. These holding fibers hold the layers together.
Therefore needling of the three layers on the needle loom in the manner as described in the above mentioned U.S. Pat. No. 5,041,330 provides a mechanical coherence of the three layers of the sealing mat and at the same time obviates the adhesive used according to the above mentioned U.S. Pat. No. 4,501,788.
Moreover, as the bentonite swells when moistened, the needling bond provides a pressure counteracting the swelling pressure which, in combination with the above described intensive mixing of fibers and bentonite, provides water impermeability of said sealing mats. Furthermore, needling ensures flexibility of the sealing mats to a degree that comes close to the good pliability properties of mechanically bonded non-woven textile materials.
Although, as the above statements show, the sealing mats according to U.S. Pat. No. 5,041,330 are in many aspects much better than the other sealing mats of the prior art, it has been found, that the transfer of shearing forces from the upper cover layer to the substrate layer should be improved.
Therefore, a primary object of the present invention is to provide a water and/or oil-impermeable sealing mat containing a swellable clay, preferably bentonite, said sealing mat being constructed such that in practical application on construction-sites, especially on slopes it can transfer such shearing forces caused by the sand and rock loads from the upper cover layer through the swollen intermediate bentonite layer directly onto the substrate layer. In other words, the water and/or oil-impermeable sealing mat according to the invention should be securely bonded even after the swelling of the bentonite, to prevent the sand or gravel filling weighted down with gravel or rocks from sliding off.
Starting from this, a further object of the present invention is to provide such a water and/or oil-impermeable sealing mat, which can be easily and econonomically manufacand mass produced.
A still further object of the present invention is to provide a water and/or oil-impermeable sealing mat, which can be manufactured in varying thickness for different applications as a water barrier, which prevents the seepage of water and/or the leaching of contaminants from ponds, reservoirs, dams, municipal and industrial waste lagoons or the like.